The present invention relates to a magnetic recording medium, which has high electromagnetic transfer characteristics and high durability and comprises a magnetic layer having ferromagnetic powder and a binder dispersed in it, and the magnetic layer is placed on a non-magnetic support.
The magnetic recording medium is widely used in such applications as recording tape, video tape, floppy disk, etc. The magnetic recording medium comprises a magnetic layer having ferromagnetic powder and a binder dispersed in it, and the magnetic layer is laminated on a non-magnetic support.
The magnetic recording medium must be at high level in such characteristics as electromagnetic transfer characteristics, running durability, and running performance. Specifically, an audio tape for reproducing musical sound must have an ability to reproduce original sound with higher quality. A video tape must have high electromagnetic transfer characteristics such as high ability to reproduce original image or picture. The magnetic recording medium used in storage device of a computer must be able to perform high density recording with high reliability.
In addition to high electromagnetic transfer characteristics as described above, the magnetic recording medium must have good running durability. To ensure good running durability, abrasive material and lubricant are generally added in the magnetic layer.
Because of sliding and contact between the medium and a magnetic head in the device where the magnetic recording medium is used, low molecular components in the binder of the magnetic recording medium tend to migrate toward surface of the magnetic layer, and this often causes problem of magnetic head contamination when these components are attached on the magnetic head.
Contamination of the magnetic head causes deterioration of electromagnetic transfer characteristics. In particular, in the device for high density recording, number of revolutions of the magnetic head is high. In a digital video tape recorder, number of revolutions of the magnetic head is as high as 9,600 rpm, and this is much higher compared with 1,800 rpm of an analog video tape recorder for household use, and 5,000 rpm of a video tape recorder for business use. Sliding speed between the magnetic recording medium and the magnetic head is getting increasingly higher, and the magnetic head and the thin film head are designed in more and more compact size, and there are strong demands to minimize and to improve the problems of magnetic head contamination caused by components of the magnetic recording medium. Also, the magnetic recording medium used for high density recording to replace floppy disk must have high strength and high reliability.
To solve the above problems and to improve property, hard binder is used to increase hardness of the magnetic layer.
For example, according to JP-A-01263927, there are provided a first and a second magnetic layers, and a polyurethane resin comprising polyether polyol, which is selected from polyethylene glycol, polypropylene glycol or polytetramethylene glycol, is used in the first magnetic layer. However, the polyurethane resin is deposited on the surface of the second magnetic layer, and this leads to higher friction coefficient during running operation and to poor durability. Further, the polytetramethylene glycol given in the example is contained in an amount of 75 weight % or more and has low strength, and there is no description on simultaneous use with a short-chain diol.
JP-B-8260531 describes a polyurethane resin comprising polyol, which contains polytetramethylene glycol and polycaprolactone. The content of the short-chain diol is about 15 weight % when calculated from the example, and glass transition temperature (Tg) of the polyurethane resin is in the range of xe2x88x9210xc2x0 C. to +20xc2x0 C. This is lower than that of the product according to the present invention. The strength of the coating film is low, and this leads to poor durability.
Similarly, JP-A-61190717 describes a polyurethane resin, which comprises polyol containing polytetramethylene glycol and polycaprolactone and a low molecular diol of glycol ester of monohydrocarboxylic acid. This aims to obtain a product with high calender workability, and coating film is often fluidized and has low durability. There is no description on glass transition temperature (Tg). Tg of the polyurethane resin given in the example is determined as about xe2x88x9230xc2x0 C. Accordingly, the strength of the coating film is low, and durability is not satisfactory.
JP-A-08147670 describes a polyurethane resin, which comprises polyol and short-chain diol of 5 mol % or less or diamine and diisocyanate. The content of the polyol is 0 to 5 mol %, and the content of ether is low even when polytetramethylene glycol is used. Thus, it has low adsorbing property to the magnetic support, and dispersion property is not sufficient.
JP-A-08293115 (U.S. Pat. No. 5,702,821) describes the use of a polyurethane resin, which comprises a short-chain diol having cyclic structure in an amount of 17 to 40 weight % and a polyol having ether content of 1 to 5 mmol/g in the polyurethane resin in an amount of 10 to 50 weight %, but both dispersion property and smoothness are not sufficient.
As described above, the conventional type polyurethane resin used as a binder for the magnetic recording medium lacks flexibility and does not meet the requirements such as high dispersion property, smoothness and high glass transition temperature even when it is manufactured using polytetramethylene glycol and short-chain diol.
The present invention provides magnetic recording medium, which comprises at least one magnetic layer where ferromagnetic powder and a binder are dispersed, and the magnetic layer is placed on a non-magnetic support, whereby the binder comprises a polyurethane resin obtained through reaction of polytetramethylene glycol and short-chain diol having cyclic structure or short-chain diol having a branched side chain containing two or more carbon atoms, the polytetramethylene glycol is contained in the polyurethane resin in an amount of 35 to 60 weight %, the short-chain diol is contained in the polyurethane resin in an amount of 10 to 40 weight %, and glass transition temperature of the polyurethane resin is within the range of 30xc2x0 C. to 120xc2x0 C.
Also, the invention provides the magnetic recording medium as described above, wherein the short-chain diol having the cyclic structure is at least one of a group of bisphenol A, alkylene oxide addition product of bisphenol A, hydrogenated bisphenol A, alkylene oxide addition product of hydrogenated bisphenol A, cyclohexanedimethanol, or cyclohexane diol.
Further, the invention provides the magnetic recording medium as described above, wherein the short-chain diol having a branched side chain containing two or more carbon atoms is at least one selected from a group of 2-ethyl-1,3-hexanediol, 2-ethyl-2-butyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-propyl-1,3-propanediol, 2,2-dipropyl-1,3-propanediol, 2-methyl-2-propyl-1,3-propanediol, and 2,2-dibutyl-1,3-propanediol.
Also, the present invention provides a magnetic recording medium, which comprises a lower layer having ferromagnetic powder or inorganic powder and a binder dispersed in it and being placed on a non-magnetic support, and at least one layer of magnetic layers where ferromagnetic powder and a binder are dispersed, whereby at least one of the binder in the lower layer or in the magnetic layer contains the polyurethane resin described in claim 1.
Also, the invention provides the magnetic recording medium as described above, wherein the ferromagnetic powder comprises iron as main component, and the iron contains yttrium with Y/Fe ratio of 0.5 to 10 atomic %.
In the magnetic recording medium of the present invention, polyurethane resin having polytetramethylene glycol as a long-chain diol component is used as a binder in the magnetic layer or in the lower layer, and the content of the polytetramethylene glycol is decreased. By adequately combining the short-chain diol having cyclic structure, the short-chain diol having a branched side chain containing two or more carbon atoms, and an organic diisocyanate, polytetramethylene glycol having high breaking extension property and high glass transition temperature can be produced. As a result, a magnetic recording medium having strong and tough coating film capable to resist high-speed sliding can be prepared.
At the same time, by improving dispersion property of the ferromagnetic powder and the non-magnetic powder, it is possible to have the coating film with smooth surface and to produce the magnetic recording medium having high electromagnetic transfer characteristics.
Further, when it is applied in the lower layer of the multi-layer tape, the polyurethane is scarcely deposited on the surface of the upper layer, and durability can be improved.
As the short-chain diol having cyclic structure used as raw material for the polyurethane resin contained in the binder of the magnetic recording medium of the present invention, the following substances may be used: diol such as bisphenol A, hydrogenated bisphenol A, bisphenol S, bisphenol P, and ethylene oxide or propylene oxide of the above compound, or mixed addition product of ethylene oxide propylene oxide, diol such as cyclohexane dimethanol, cyclohexane diol, etc. or diol obtained by adding ethylene oxide, propylene oxide, or butylene oxide to dibasic acid such as phthalic acid, sulfo-isophthalic acid, etc.
More preferably, hydrogenated bisphenol A or ethylene oxide, or propylene oxide addition product of the hydrogenated bisphenol A may be used.
It is preferable that the short-chain diol having cyclic structure is contained in the polyurethane resin in an amount of 10 to 40 weight %. If it is contained in an amount of lower than 10 weight %, dynamic strength is decreased, leading to poor durability. If it is contained in an amount of more than 40 weight %, solubility to solvent is decreased, resulting in lower dispersion property. Also, the coating film becomes fragile, and durability is decreased.
It is preferable that the short-chain diol having a branched side chain containing two or more carbon atoms is contained in the polyurethane resin in the present invention in an amount of 10 to 40 weight %. If it is contained in an amount of lower than 10 weight %, dynamic strength is decreased, leading to poor durability. If it is contained in an amount of more than 40 weight %, solubility in solvent decreases, and dispersion property is also decreased.
As the short chain diol having a branched side chain containing two or more carbon atoms, the following substances may be used: 2-ethyl-1,3-hexanediol, 2-ethyl-2-methyl-1,3-propanediol, 2-butyl-1,3-hexanediol, 2-normalbutyl-2-ethyl-1,3-propanediol, 3-ethyl-1,5-pentanediol, 3-butyl-1,5-pentanediol, 3-propyl-1,5-pentanediol, 2,2-diethyl-1,3-propanediol, 3-octyl-1,5-pentanediol, 3-myristyl-1,5-pentanediol, 3-stearyl-1,5-pentanediol, 2-ethyl-1,3-propanediol, 2-ethyl-1,5-pentanediol, 2-ethyl-1,6-hexanediol, 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-propyl-1,3-propanediol, 2,2-dipropyl-1,3-propanediol, 2-methyl-2-propyl-1,3-propanediol, or 2,2-dibutyl-1,3-propanediol.
Among these substances, it is more preferable to use 2-ethyl-1,3-hexanediol, 2-ethyl-2-butyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-propyl-1,3-propanediol, 2,2-dipropyl-1,3-propanediol, 2-methyl-2-propyl-1,3-propanediol, or 2,2-dibutyl-1,3-propanediol. These substances can be expressed by the following formulae: 
Among these substances, it is more preferable to use 2-ethyl-2-butyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, or 2,2-dibutyl-1,3-propanediol.
The number of the carbon atoms in the branched carbon chain is preferably 2 to 10, or more preferably 2 to 6. If it is lower than 2, solubility in solvent is lowered, and dispersion property is decreased. If it is more than 10, solubility in solvent is also decreased, and the dispersion property is lowered.
The polytetramethylene glycol used as the long-chain polyol is contained in the polyurethane resin preferably in an amount of 35 to 60 weight %, or more preferably in 40 to 50 weight %. If its content is lower than 35 weight %, solubility in solvent is decreased and dispersion property is also decreased. If its content is more than 60 weight %, the strength of the coating film is decreased, leading to poor durability. Weight average molecular weight of the polytetramethylene glycol is preferably 500 to 5,000, or more preferably 1,000 to 3,000.
As the organic diisocyanate used together with the polytetramethylene glycol, the short-chain diol having cyclic structure, or the short-chain diol having the branched side chain, the following compounds may be used: aromatic diisocyanate such as 2,4-tolylene-diisocyanate, 2,6-tolylene-diisocyanate, xylene-1,4-diisocyanate, xylene-1,3-diisocyanate, 4-4xe2x80x2-diphenylmethane-diisocyanate, 4,4-diphenylether-diisocyanate, 2-nitrodiphenyl-4,4xe2x80x2-diisocyanate, 2,2xe2x80x2-diphenylpropane-4,4xe2x80x2-diisocyanate, 4,4xe2x80x2-diphenylpropane-diisocyanate, m-phenylene-diisocyanate, p-phenylene-diisocyanate, naphthylene-1,4-diisocyanate, naphthylene-1,5-diisocyanate, 3,3xe2x80x2-dimethoxydiphenyl-4,4xe2x80x2-diisocyanate, etc., aliphatic diisocyanate such as tetramethylene diisocyanate, hexamethylene diisocyanate, lysine diisocyanate, etc. or cycloaliphatic diisocyanate such as isophorone-diisocyanate, hydrogenated tolylene-diisocyanate, hydrogenated diphenylmethane diisocyanate, etc.
It is preferable to use aromatic diisocyanate, or more preferably 4,4-diphenylmethane diisocyanate, 2,2-tolylene diisocyanate, p-phenylene diisocyanate, isophorone diisocyanate.
Weight average molecular weight (Mw) of the polyurethane resin used in the magnetic recording medium of the present invention is preferably 30,000 to 70,000, or more preferably 40,000 to 60,000. If it is less than 30,000, strength of the coating film is decreased, leading to poor durability. If it is more than 70,000, solubility in solvent is lowered, leading to poor dispersion property.
Glass transition temperature (Tg) of the polyurethane resin used in the magnetic recording medium of the present invention is preferably 30xc2x0 C. to 120xc2x0 C., or more preferably 50xc2x0 C. to 90xc2x0 C. If it is lower than 30xc2x0 C., strength of the coating film is decreased at high temperature, thus resulting in lower durability and storage property. If it is higher than 120xc2x0 C., calender workability is decreased, leading to poor electromagnetic transfer characteristics.
It is preferable that the polyurethane resin contains a polar group. As the polar group of the polyurethane, xe2x80x94SO3M, xe2x80x94OSO3M, xe2x80x94PO3M2, or xe2x80x94COOM are preferably used, or more preferably, xe2x80x94SO3M or OSO3M. It is preferable that the content of the polar group is 1xc3x9710xe2x88x925 eq/g to 2xc3x9710xe2x88x924 eq/g. If it is less than 1xc3x9710xe2x88x925 eq/g, adsorption to magnetic substance becomes insufficient, and this leads to poor dispersion property. If it is more than 2xc3x9710xe2x88x924 eq/g, solubility in solvent is decreased, leading to poor dispersion property.
The content of OH groups in the polyurethane resin exerts influence on physical property of the binder to be obtained. Preferably, 2 to 20 OH groups are contained per molecule in the polyurethane resin, or more preferably 3 to 15 groups per molecule.
If it is less than 3 groups per molecule, reactivity with isocyanate curing agent is decreased, and this reduces the strength of the coating film and leads to poor durability. On the other hand, if it is 15 groups or more per molecule, solubility in solvent decreases, leading to poor dispersion property.
As the ferromagnetic powder, Fe, Fexe2x80x94Co, Fexe2x80x94Ni, or Coxe2x80x94Nixe2x80x94Fe containing yttrium may be used. The content of yttrium in the ferromagnetic powder is preferably such that the ratio of yttrium atoms to iron atoms (Y/Fe) is 0.5 to 20 atomic %, or more preferably 5 to 10 atomic %. If it is lower than 0.5 atomic %, it is not possible to have ferromagnetic powder with higher "sgr" s value. As a result, magnetic characteristics decrease, leading to poor electromagnetic transfer characteristics. If it is higher than 20 atomic %, the iron content is decreased. This leads to lower magnetic characteristics and poor electromagnetic transfer characteristics. Further, any of the following elements may be contained within the range of not more than 20 atomic % to 100 atomic % of iron: aluminum, silicon, sulfur, scandium, titanium, vanadium, chromium, manganese, copper, zinc, molybdenum, rhodium, palladium, gold, tin, antimony, boron, barium, tantalum, tungsten, rhenium, gold, lead, phosphorus, lanthanum, cerium, praseodymium, neodymium, tellurium, bismuth, etc. Also, the ferromagnetic metal powder may contain small quantity of water, hydroxide or oxide.
In the ferromagnetic powder of the present invention, in addition to yttrium, elements such as neodymium, samarium, praseodymium, lanthanum, etc. may be introduced. These can be introduced using chloride such as yttrium chloride, neodymium chloride, samarium chloride, praseodymium chloride, lanthanum chloride, etc. or nitrate such as neodymium nitrate, gadolinium nitrate, etc. Two or more types of these compounds may be simultaneously used.
There is no special restriction on the shape of the ferromagnetic powder. In general, the ferromagnetic powder in needle-like, particulate, cubic, grain-like or planar shape may be used. In particular, it is preferable to use needle-like ferromagnetic powder.
The resin components, the curing agent, and the ferromagnetic powder as given above are kneaded and dispersed together with methylethylketone, dioxane, cyclohexanone, ethyl acetate, etc. used in the preparation of magnetic coating material, and a magnetic coating material is prepared. The kneading and dispersing may be performed according to the procedure commonly known.
The magnetic coating material may contain, in addition to the components given above, abrasive material such as xcex1xe2x80x94Al2O3, Cr2O3, etc., anti-static agent such as carbon black, lubricant such as fatty acid, fatty acid ester, silicone oil, etc., or additive or filling agent normally used such as dispersing agent.
Next, description will be given on the lower non-magnetic layer or the lower magnetic layer in case the medium of the present invention is designed in multi-layer structure. Inorganic powder in the lower layer in the present invention may be magnetic powder or non-magnetic powder. For example, in case of non-magnetic powder, inorganic powder can be selected from inorganic compound such as metal oxide, metal carbonate, metal sulfate, metal nitride, metal carbide, metal sulfide, etc. As the inorganic compounds, the following substances are used in combination or alone: For example, xcex1-alumina with alpha ratio of 90%-100% or more, xcex2-alumina, xcex3-alumina, silicon carbide, chromium oxide, cerium oxide, xcex1-iron oxide, corundum, silicon nitride, titanium carbide, titanium oxide, silicon dioxide, tin oxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide, etc. In particular, it is preferable to use titanium dioxide, zinc oxide, iron oxide or barium sulfate, or more preferably titanium dioxide. Average particle size of the non-magnetic powder is preferably in the range of 0.005 to 2 xcexcm. When necessary, non-magnetic powder with different average particle sizes may be mixed together or similar effect can be provided with single type of non-magnetic powder by widening the particle size distribution. In particular, it is preferable that average particle size of the non-magnetic powder is 0.01 to 0.2 xcexcm. It is preferable that pH value of the non-magnetic powder is within the range of 6 to 9. Specific surface area of the non-magnetic powder is preferably 1 to 100 m2/g, or more preferably 5 to 50 m2/g, or most preferably 7 to 40 m2/g. Crystallite size of the non-magnetic powder is preferably 0.01 to 2 xcexcm. Oil absorption using DBP is preferably 5 to 100 ml/100 g, or more preferably 10 to 80 ml/100 g, or most preferably 20 to 60 ml/100 g. Specific gravity is preferably 1 to 12, or more preferably 3 to 6. The shape may be any of needle-like, spherical, polyhedron, or planar shape.
It is preferable that surface of the non-magnetic powder is processed by surface treatment using Al2O3, SiO2, TiO2, ZrO2, SnO2, Sb2O3, or ZnO. To ensure better dispersion property, it is preferable to use Al2O3, SiO2, TiO2 or ZrO2, or more preferably Al2O3, SiO2 or ZrO2. These substances may be used in combination or alone. According to each individual purpose, coprecipitated surface treatment layer may be used, or a method to treat the surface layer using silica after treating with alumina, or a method reversing this procedure may be adopted. The surface treatment layer may be porous according to the purpose. In general, it is preferably homogeneous and dense.
By mixing carbon black in the lower layer, it is possible to decrease the value of Rs as the effect already known and to attain micro Vickers hardness as desired. For this purpose, furnace black for rubber, thermal black for rubber, carbon black for color, acetylene black, etc. may be used.
Specific surface area of the carbon black is preferably 100 to 500 m2/g, or more preferably 150 to 400 m2/g. DBP oil absorption is preferably 20 to 500 ml/100 g, or more preferably 30 to 400 ml/100 g. Average particle size of the carbon black is preferably 5 to 80 xcexcm, or more preferably 10 to 50 xcexcm, or most preferably 10 to 40 xcexcm. In the carbon black, it is preferable that pH value is 2 to 10, moisture content is 0.1 to 10%, and tap density is 0.1 to 1 g/ml. Concrete examples of the carbon black to be used in the present invention are as follows: Blackpearls 2000, 1300, 1000, 900, 800, 880, 700, and Vulcan XC-72 (manufactured by Cabot), #3050B, #3150B, #3250B, #3750B, #3950B, #950, #650B, #970B, #850B, and MA-600 (manufactured by Mitsubishi Chemical Industry Co., Ltd.), Conductex SC, Raven 8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255, and 1250 (manufactured by Columbia Carbon Co.), and Ketchenblack EC (manufactured by Akzo).
In the lower layer of the present invention, ferromagnetic powder may also be used. As the ferromagnetic powder, an alloy having xcex3-Fe2O3, Co-denatured xcex3-Fe2O3, or xcex1-Fe as principal component, or CrO2 may be used. In particular, it is preferable to use Co-denatured xcex3-Fe2O3. The ferromagnetic powder used in the lower layer of the present invention has preferably composition and performance characteristics similar to those of the ferromagnetic powder used in the upper magnetic layer. However, the performance characteristics may be varied in the upper and the lower layers as already known according to each individual purpose. For example, to improve long wavelength recording characteristics, it is desirable to set the value of Hc of the lower magnetic layer to a value lower than that of the upper magnetic layer. Also, it is more effective to set the value of Br in the lower magnetic layer to a value higher than that of the upper magnetic layer. Further, advantages can be provided by adopting a multi-layer structure as already known.
As binder, lubricant, dispersing agent, additive, solvent, dispersing method, etc. of the lower magnetic layer or the lower non-magnetic layer, those for the magnetic layer can be applied. In particular, for quantity and type of binder, and quantity and type of additive and dispersing agent, the technique already known for the magnetic layer may be applied.
The magnetic coating material prepared from the above materials is coated on the non-magnetic support, and a magnetic layer is formed.
As the non-magnetic support used in the present invention, polyethylene naphthalate, polyethylene terephthalate, polyamide, polyimide, polyamideimide, aromatic polyamide, polybenzoxydazole, etc. processed by biaxial stretching may be used. More preferably, polyethylene naphthalate or aromatic polyamide may be used. These non-magnetic supports may be processed in advance by corona discharge, plasma treatment, treatment to make more easily adhesive, heat treatment, etc. The non-magnetic support used in the present invention has preferably such surface smoothness that average surface roughness at central line is in the range of 0.1 to 20 nm under cut-off value of 0.25 mm, or more preferably in the range of 1 to 10 nm. It is also preferable that the non-magnetic support not only has lower average surface roughness at central line but also has no excessive projection of more than 1 xcexcm.
The magnetic recording medium of the present invention can be manufactured, for example, by the following method: On the surface of a non-magnetic support under running condition, a coating solution for the magnetic layer is coated so that thickness of the magnetic layer after drying will be within the range of 0.05 to 3.0 xcexcm, or more preferably 0.07 to 1.0 xcexcm. In this case, a plurality of magnetic coating materials may be coated in multiple layers sequentially or at the same time.
On the surface (where the coating material is not coated) of the non-magnetic support used in the present invention, a back-coating layer (backing layer) may be provided. The back-coating layer is a layer, which is produced by coating a coating material to form back-coating layer (i.e. a coating material where particulate components such as abrasive material, anti-static material, etc. and the binder are dispersed in an organic solvent) on the uncoated surface of the non-magnetic support.
An adhesive layer may be provided on the surface of the non-magnetic support coated with the magnetic coating material and also with the coating material to form back-coating layer.
The coating layer coated with the magnetic coating material is dried after magnetic field orientation processing is performed on the ferromagnetic powder contained in the coating layer of the magnetic coating material.
After it has been dried as described above, surface smoothening treatment is carried out on the coating layer. For the surface smoothening treatment, super calender roll is used, for example. By the surface smoothening treatment, the holes generated due to removal of the solvent during drying are eliminated, and filling ratio of the ferromagnetic powder in the magnetic layer is improved. This makes it possible to obtain a magnetic recording medium having high electromagnetic transfer characteristics.
As the calender processing roll, heat-resistant plastic roll made of epoxy, polyimide, polyamide, polyamideimide, etc. is used. Or, a metal roll may be used.
It is preferable that the magnetic recording medium according to the present invention has surface with such good smoothness that average roughness along the central line of the surface is preferably within the range of 0.1 to 4 nm under the cut-off value of 0.25 mm, or more preferably within the range of 1 to 3 nm. To attain such surface smoothness, calender treatment is performed on the magnetic layer, which is formed by selecting specific types of the ferromagnetic powder and the binder as described above. The conditions for the calender treatment are as follows: Temperature of calender roll is preferably within the range of 60 to 100xc2x0 C., or more preferably 70 to 100xc2x0 C., or most preferably 80 to 100xc2x0 C. Pressure is preferably within the range of 100 to 500 kg/cm2, or more preferably 200 to 450 kg/cm2, or most preferably 300 to 400 kg/cm2.
The laminated substance processed by curing treatment as described above is cut into the shape as desired.
In the magnetic recording medium obtained as described above, the content of the polytetramethylene glycol is lower than that of the conventional type polyurethane resin. Also, by combining the short-chain diol having cyclic structure or the short-chain diol having branched side chain, it is possible to provide the polyurethane resin containing the polytetramethylene glycol with high breaking extension property. Because the content of the short-chain diol is higher than in the conventional type polyurethane resin, urethane bonding concentration is increased substantially and high glass transition temperature can be attained. This makes it possible to obtain strong and tough coating film capable to endure high-speed sliding operation. When it is applied in the lower layer of a multi-layer type, the polyurethane resin is scarcely deposited on the surface of the upper layer when the coating film is dried, and durability is improved.
(Embodiments)